1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device employing an organic insulating layer. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for increasing an aperture ratio of the liquid crystal display device.
2. Discussion of the Related Art
Recently, with the advent of the information age, a display field for processing and displaying mass information has been developed. Until recently, cathode-ray tubes (CRT) have been the main stream of display device and have been developed continuously.
Meanwhile, a flat panel display is increasingly in demand so as to meet the requirements of compact size, lightweight, low power consumption, and the like. Accordingly, a thin film transistor-liquid crystal display (hereinafter, referred to as TFT-LCD) having an excellent color reproduction and a slim size has been developed.
The operation of a TFT-LCD will be described below. If a selected pixel is switched by a thin film transistor, the switched pixel can adjust the amount of light transmission of a lower light source.
An amorphous silicon thin film transistor (a-Si:H TFT) having a semiconductor layer formed of amorphous silicon is usually used as a switching device. This is because the amorphous silicon thin film can be formed at a low temperature on a large-sized insulating substrate, such as a glass substrate.
In the TFT-LCD that is widely used, a backlight unit disposed at a lower portion of the panel has employed a method of displaying images by using the light emitted from the backlight unit.
However, the TFT-LCD is a very inefficient optical modulator because it transmits only 3% to 8% of the light that is emitted from the backlight unit.
For example, when the transmittance of two polarizing plates is 45%, the transmittance of two glasses is 94%, the transmittance of a TFT array and a pixel is about 65%, and the transmittance of a color filter is about 27%, the transmittance of the TFT-LCD would be about 7.4%.
FIG. 1 is a plane view of a pixel structure of a related art liquid crystal display device provided.
Referring to FIG. 1, a plurality of gate bus lines 1 and data bus lines 3a and 3b perpendicularly cross one another on a transparent insulating substrate and defining a plurality of unit pixel regions.
A switching thin film transistor (hereinafter, referred to as TFT) including a gate electrode 7, a source electrode 5a, and a drain electrode 5b is arranged on a region where the gate bus line 1 and the data bus lines 3a and 3b perpendicularly cross one another. A pixel electrode 9a is contacted with the drain electrode 5b of the TFT and parallel with the data bus lines 3a and 3b is arranged on the unit pixel region.
In order to increase an aperture ratio, the pixel electrode 9a overlaps predetermined portions of the left and right data bus lines 3a and 3b defining the unit pixel region.
A process of driving the LCD with the pixel electrode 9a formed to increase an aperture ratio will be described below.
If a driving signal is applied through the gate bus line 1, the TFT arranged in the unit pixel region is turned on. At this point, a graphic signal applied through the data bus line 3a is transferred to the pixel electrode 9a through the TFT.
An electric field is generated due to the graphic signal applied to the pixel electrode 9a. The electric field causes the liquid crystal molecules of the liquid crystal molecules to be twisted. Also, due to the electric field, the transmittance progressed from the backlight is adjusted to reproduce R, G, and B colors.
FIG. 2 is a plane view of a color filter structure of the related art liquid crystal display device.
Referring to FIG. 2, a black matrix 15 formed on the color filter substrate defines the unit pixel regions, and R, G, and B color filter layers 10 are formed on each unit pixel region, respectively. Here, the black matrix 15 is formed by patterning a chrome metal layer so as to arrange the black matrix to correspond to the unit pixel of the array substrate.
In the black matrix 15, a non-transmission region or a disclination region of the unit pixel region formed on the array substrate is isolated and patterned so as to reproduce R, G, and B colors.
As described above, the black matrix 15 defines a space for the color filter layer on the color filter substrate so as to arrange the black matrix 15 to correspond to the unit pixels of the array substrate. Then, the R, G, and B color filter layers 10 are formed on the unit pixel region.
In order to isolate the non-transmission region and the light leakage occurrence region, which are arranged at the unit pixel region of the array substrate, the black matrix 15 blocks each intersection between the gate bus line and the data bus line and the region corresponding to the TFT region.
FIG. 3 is a cross-sectional view of the pixel structure of the related art liquid crystal display device.
Referring to FIG. 3, the R, G, and B color filter layers 10 are arranged on the color filter substrate, and the gate bus line, the data bus line 13, and the switching TFT are arranged on the array substrate. The color filter substrate and the array substrate are attached to each other, so that the liquid crystal layer 18 is interposed therebetween.
As shown in FIG. 3, the black matrix 15 arranged on the color filter substrate and the R, G, and B color filter layer 10 correspond to the pixel electrodes 15a and 15b and the data bus lines 13, respectively.
In order to increase an aperture ratio, an organic insulating layer 14 is used, and the pixel electrodes 15a and 15b arranged on the unit pixel region of the array substrate overlap predetermined portions ‘a’ and ‘b’ of the data bus lines 13.
Additionally, the black matrix 15 arranged on the color filter substrate has a width overlapping the data bus lines 13 formed on the array substrate.
As shown in FIG. 3, in order to increase an aperture ratio, a desirable width of the pixel electrode 15a arranged on the array substrate overlaps the data bus line 13 disposed at a boundary region of the unit pixel together with an adjacent pixel electrode 15b. On the other hand, the pixel electrode 15a of the pixel region having the TFTs arranged along the data bus lines 13 overlaps a much wider area of the pixel electrode 15b connected with the TFT of the adjacent data bus line 13.
However, in such an LCD device employing the organic insulating layer to enhance the aperture ratio, a light leakage occurs at a banding edge portion and a pixel edge portion due to an abnormal alignment of liquid crystals, resulting in a disclination. Here, the banding edge portion and the pixel edge portion overlap the gate bus line and the data bus line due to the expansion of the pixel electrode.
Additionally, although the overlapping regions are formed only between the TFT of the array substrate and the data bus line and between the gate bus line and the pixel electrodes so as to increase an aperture ratio, the use of the organic insulating layer causes a light leakage due to a variation of the cell gap.
Further, since the overlapping region is determined while taking only the TFT of the array substrate into consideration, a light leakage may be caused due to a movement of the upper black matrix according to a variation of attachments between the upper and lower substrates.
As described above, the width of the black matrix is expanded so as to prevent a light leakage. However, this causes a problem of reducing an aperture ratio.